While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep UV and EUV lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is thought requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F2 laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems. A highlight was suddenly placed on the ArF immersion lithography that introduces a liquid having a higher refractive index than air (e.g., water, ethylene glycol, glycerol) between the projection lens and the wafer and that allows the projection lens to be designed to a numerical aperture (NA) of 1.0 or higher and achieves a higher resolution. See Journal of Photopolymer Science and Technology, Vol. 17, No. 4, p 587 (2004).
In the photolithography using an ArF excimer laser (wavelength 193 nm), a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing high sensitivity resist material, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, polymers of acrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene, ring-opening metathesis polymerization (ROMP) polymers, and hydrogenated ROMP polymers have been proposed as the base resin. This choice is effective to some extent in that the transparency of a resin alone is increased.
Studies have also been made on photoacid generators. In prior art chemically amplified resist compositions for the lithography using KrF excimer laser, photoacid generators capable of generating alkane- or arene-sulfonic acids are used. However, the use of these photoacid generators in chemically amplified resist compositions for ArF lithography encounters several drawbacks including an insufficient acid strength to scissor acid labile groups on the resin, a failure of resolution, and a low sensitivity. Thus these photoacid generators are not suited for the fabrication of microelectronic devices.
For the above reason, sulfonium salts comprising a triphenylsulfonium cation and a perfluoroalkanesulfonic acid anion are generally used as the photoacid generator in ArF chemically amplified resist compositions. Among perfluoroalkanesulfonic acids, perfluorooctanesulfonic acid (PFOS) is difficult to apply to resist materials because of its risks of difficult degradation, biological concentration and toxicity. Although photoacid generators capable of generating perfluorobutanesulfonic acid are used in the existing resist materials, it is still difficult to provide a high resolution because the acid generated therefrom are highly diffusible.
The lithography techniques which are considered promising next to the ArF lithography include electron beam (EB) lithography, F2 lithography, extreme ultraviolet (EUV) lithography, and x-ray lithography. In these techniques, exposure must be done in vacuum or reduced pressure, which allows the sulfonic acid generated during exposure to volatilize, failing to form a satisfactory pattern profile. Volatile sulfonic acids and volatile photolysis products of sulfonium cations (e.g., phenylsulfides) become so-called outgases which can damage the exposure system. Exemplary photolysis is shown below.
Herein Y—H designates a proton donor such as a polymer matrix. This photolysis product is exemplary while more complex photolysis products can be formed. The attempt to reduce the outgassing from acid generators is described, for example, in JP-A 2009-037057.